Honestly, there’s no one given answer.
Some designers solely design for women, so their numbers are going to be smaller. Others choose to focus on abstract designs. Some exclusively do animated designs.
I would say aim to improve. Meaning as you get better, you’ll get more attention, be it from Facer or followers. If you get lucky a design will get featured which usually results in lots of syncs/favorites. It can get featured on Facers instagram, or on the actual website.
So, based on that, as you get more followers, you should get more syncs/favorites. If you find those numbers disappear you may want to try something new design wise. What you think is popular may not and vice versa.
If you want to become a premium designer, you’re aiming for 3000 syncs in a month (which is doable with followers and getting featured)… for 3 different watch faces. So don’t set that as a goal for success or failure, in other words.
Other goals? It’s fun to get comments on your watch. For people to take “wrist selfies” etc.
My own personal goal is to have a follower for each watchface I’ve designed.
My advice would be to design a watchface for each category. Get a feel for the different type audiences out there. If you aren’t promoting yourself (I haven’t) on forums then you should consider themes. For example, I like pop culture, so I design a watch for a movie in anticipation of people searching for a watchface. As the movie nears, I notice that watchface gets a LOT more syncs. Try partnering up with designers… we try to challenge each other sometimes. We had a lot of people do Star Wars faces for May the Fourth… we’re doing it again for the 4th of July.
It’s cheesy but design for yourself and you’ll always be happy. Anecdotally, realistic watch designs seem to be popular. I’ve had some “success” with unique designs and pop culture stuff. Look at the Top 100 list (Facer - Thousands of FREE watch faces for Apple Watch, Samsung Gear S3, Huawei Watch, and more) and Trending (Facer - Thousands of FREE watch faces for Apple Watch, Samsung Gear S3, Huawei Watch, and more) to see what is popular at the moment.
Sorry for the jumbled mess of thoughts. Hope it helps!